The present invention relates to a laser beam scanning device for deflecting a laser beam in order to scan the same on a photosensitive medium.
FIG. 2 is a schematic view of the conventional laser beam scanning system utilized in a laser printer. As shown, the laser beam scanning system uses, as a laser beam source a semiconductor laser 1. By turning the source on and off in response to a laser modulation, signal inputted according to an image signal the generation of the laser beam can be controlled. The laser beam scanning system is further comprised of; a collimator lens 2 for collimating the laser beam emitted from the semiconductor laser 1 into a substantially parallel laser beam, a polygon mirror 3 rotatable at a predetermined rotational number to deflect the laser beam passed from the collimator lens 2 in a primary scanning direction, an f.theta. lens 5 disposed between the polygon mirror 3 and a photosensitive drum 4 for converging the laser beam reflected by the polygon mirror 3 onto the photosensitive drum 4, a pair of preceding cylindrical lens 6 disposed between the collimator lens 2 and the polygon mirror 3 and a succeeding cylindrical lens 7 disposed between the f.theta. lens 5 and the photosensitive drum 4 for compensating for inclinations of reflective surfaces of the polygon mirror, and a photodetector 8 for detecting a starting position where the laser beam begins to write in the primary scanning direction, passed from the succeeding cylindrical lens 7. The f.theta. lens 5 and cylindrical lens 7 constitute an optical converging system for converging the reflected beam from the polygon mirror onto the surface of photosensitive medium. A polygon mirror driving system is comprised of; a polygon motor 9 of the DC motor type, a combination of a rotary encoder 13 and a photointerrupter 14 for detecting a rotation number of the motor 9, and a polygon motor driving circuit 15.
A secondary scanning system functions to rotate the photosensitive drum. In similar manner to the polygon mirror driving system, the secondary scanning system is comprised of a DC motor 10 for rotating the photosensitive drum 4, the combination of a rotary encoder 11 and a photointerrupter 12 for detecting a rotation number of the DC motor 10, and a photosensitive drum motor rotating or driving circuit 16.
Since both the polygon motor 9 and photosensitive drum motor 10 need to control their rotation number, separate, conventional rotation number controlling circuits are provided in the respective driving circuits. Since the polygon motor driving circuit 15 and the photosensitive drum motor rotating circuit 16 have the same basic structure, the polygon motor driving circuit 15 will be explained for example hereinbelow.
FIG. 3 is a block diagram of the conventional polygon motor driving circuit. In FIG. 3, an oscillating source circuit 91 utilizes a quartz resonator, and an output signal thereof is inputted into a frequency-dividing circuit 92. On the other hand, the photointerrupter 14 cooperates with the rotary encoder 13 directly connected to a shaft of the motor 9 to output a detection signal in synchronization with the rotation of the motor. The detection signal is input to a waveform shaping circuit 96 effective to remove distortion of a waveform of the detection signal, and thereafter an output signal of the waveform shaping circuit 96 is input to a phase comparator 93 to effect phase comparison with respect to the output signal from the frequency-dividing circuit 92. An output signal of the phase comparator 93 is input to an integral circuit 94 to change the same into a voltage signal. The voltage signal of the integral circuit 94 is input to an amplifying circuit 95 to produce the electric power needed for driving the motor. Then the output power of the amplifying circuit 95 is applied to the motor 9. Such circuit structure is known generally as a Phase-Locked Loop (PLL) control circuit. Consequently, in the conventional laser beam scanning device, a pair of PLL control circuits are provided independently from each other for controlling the rotation number of the polygon motor 9 and the DC motor 10 operative to rotate the photosensitive medium, separately from each other.
The overall operation of the conventional device will be briefly described with reference to FIG. 2. First, the polygon mirror 3 is rotated with a given rotation number by means of the polygon motor driving circuit 15. The rotation number of the polygon mirror 3 is monitored by means of the combination of slits cut on a circumference of the rotary encoder 13 directly connected to the shaft of the motor 9 and the photointerrupter 14. Namely, the photointerrupter produces a pulsed, detection signal having a frequency proportional to the rotation number of the motor 9. Then, the pulsed signal is phase-compared with the reference clock signal from the frequency dividing circuit in the polygon motor driving circuit 15 so as to effect the PLL control to rotate the motor 9 at a constant rotation number. Next, cleaning and corona charging are carried out for preparation of image writing on the photosensitive medium.
Thereafter, in order to rotate the photosensitive drum 4 at a predetermined secondary scanning speed, the photosensitive drum rotating circuit 16 effects PLL control of the rotation number of the DC motor 10.
Then, when the photosensitive drum 4 comes to a vicinity of a predetermined angular position, the semiconductor laser is turned on and the polygon mirror 3 reflects the laser beam to scan the laser beam in the primary scanning direction. After a given time interval from a time when the scanned laser beam crosses the photosensor 8, the semiconductor laser is turned on and off according to an inputted image signal to effect writing of the first line of the image. During the writing of the first line, the photosensitive drum 4 continues to rotate at the constant rotation number and the photodetector 8 detects the starting position of writing in the primary scanning direction. Therefore, while aligning the starting position of writing of each line, the second and subsequent lines are written on the drum without duplication to thereby reproduce the two-dimensional image. The rotation speed of the photosensitive drum 4 is PLL-controlled so as to equalize a pitch of the respective lines. By carrying out this writing operation for all of the lines or rows, the desired two-dimensional image can be reproduced on the photosensitive drum 4.
However, the laser beam scanning system of the above described structure operates based on the rotation number control of the photosensitive drum driving system and does not have a function to directly control the angular position of the drum. Further, the rotation number control system of the polygon mirror and the rotation number control system of the photosensitive drum are operated independently of each other so that the conventional device has drawbacks or problems such as: (1) the starting position of each line of writing is not accurately aligned in the direction of the secondary scanning, stated otherwise the pitch of the lines is not sufficiently accurate as to enable the precise color image reproduction, and (2) it is not possible to start writing of each line from a desired position of the secondary scanning, stated otherwise each position of the lines is not changed or adjusted for various printing condition.